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    关系数据库 - DSRBLOG
    
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						关系数据库
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								<p>
									<span class="date">2022/09/23 16:44 下午</span>
									

									

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					<h2><a id="%E5%85%B3%E7%B3%BB%E6%A8%A1%E5%9E%8B%E7%9A%84%E6%95%B0%E6%8D%AE%E7%BB%93%E6%9E%84%E5%8F%8A%E5%85%B6%E5%BD%A2%E5%BC%8F%E5%8C%96%E5%AE%9A%E4%B9%89" class="anchor" aria-hidden="true"><span class="octicon octicon-link"></span></a>关系模型的数据结构及其形式化定义</h2>
<h3><a id="%E5%9F%9F" class="anchor" aria-hidden="true"><span class="octicon octicon-link"></span></a>域</h3>
<ul>
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<p><strong>域</strong>是一组具有<strong>相同数据类型</strong>的值的<strong>集合</strong>，又称为<strong>值域</strong></p>
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<p>域中所包含的值的个数称之为域的<strong>基数</strong>（用m表示）</p>
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<pre class="line-numbers"><code class="language-SQL">    D1 = {李力， 王平，刘伟}, m1 = 3;
    D2 = {男, 女}; m2 = 2;
    D3 = {18, 20, 18}; m3 = 3;
</code></pre>

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